There has been continuing effort to develop new and improved expansion methods for increasing the filling capacity of tobacco. Numerous methods for expanding tobacco have been proposed. While some methods involve the use of heat, steam and/or pressure, freeze drying, and the like, others involve the use of various substances in gaseous, liquid or solid form, which are applied on or impregnated into the tobacco matrix, and thereafter the impregnated tobacco is subjected to heat whereby the tobacco is expanded. Illustrative of tobacco expansion technology are the disclosures of British Pat. No. 1,331,640 and U.S. Pat. No. 3,771,533 patents.
A recently proposed process for expanding tobacco involves applying finely powdered ammonium carbamate to cut tobacco, and after a holding period at ambient temperature and pressure for 4-24 hours, the tobacco matrix is heated to effect substantial expansion of the tobacco. Optimal results are dependent on the treatment of the cut tobacco with high purity ammonium carbamate, preferably in a finely powdered free-flowing form.
High purity ammonium carbamate in the form of a fine powder is not a commercially available product. Efforts to purify and grind commercial ammonium carbamate are inefficient and uneconomical, particularly since ammonium carbamate is sensitive to the presence of moisture. Inorganic Syntheses, 2, 85-86(1946) describes ammonium carbamate as slightly volatile at room temperature and completely dissociated at 138.degree. F. It is very soluble in water but, in solution or on standing in moist air, it undergoes hydration to ammonium carbonate: EQU H.sub.2 NCO.sub.2 NH.sub.4 +H.sub.2 O .fwdarw.(NH.sub.4) .sub.2 CO.sub.3
It is known that ammonium carbamate can be formed by interacting carbon dioxide and ammonia in the form of gases and/or liquids and/or solids. Due to the strong exothermic heat of formation and the low temperature of decomposition of the product, a high yield of pure ammonium carbamate is not readily obtained in an economically practical manner.
German Auslegeschrift No. 1,097,430 (Jan. 19, 1961) proposes a process for improving the yield and purity of ammonium carbamate as prepared by the interaction of liquid carbon dioxide and liquid ammonia. Finely powdered ammonium carbamate is obtained by releasing the two liquid reactants from a mixing nozzle into a closed space, with one of the reactants being present in a stoichiometric excess.
In practice, the blending of liquid carbon dioxide and liquid ammonia in a mixing nozzle for spraying into an enclosed space involves difficulties which hamper a continuous mode of operation. First, clogging of lines is caused by ice formation due to the presence of moisture in the reactants. Second, there is periodic clogging of the mixing nozzle chamber and outlet, because upon contact of the carbon dioxide and ammonia there is instantaneous formation of particulate ammonium carbamate within the confines of the mixing nozzle.
There remains a need for improved apparatus and means for producing ammonium carbamate in an economically feasible manner.
Accordingly, it is an object of this invention to provide a novel dispensing assembly adapted for delivering two separate instantaneously coreactive liquid streams into an external contacting zone, which dispensing assembly can operate continuously over an extended period without internal clogging.
It is a further object of this invention to provide a process for producing free-flowing ammonium carbamate powder having a purity of substantially 100 percent.
Other objects and advantages of the present invention shall become apparent from the accompanying description and drawings.